We have previously demonstrated the potential of RB-mediated tumor suppression. Direct test of the hypothesis that RB gene therapy is effective in the context of the whole organism is hindered by the lack of suitable animal models that faithfully mimic human RB-deficient cancers. In particular, RB plus/minus mice die prematurely due to brainstem compression by melanotroph tumors. Such tumors are extremely rare in humans. To resolve this difficulty and to explore the efficacy of RB therapy, we intend to generate mouse models that closely parallel human cancer by refining current model systems and by creating new ones. Three specific aims are proposed: Aim 1 is to modify the spectrum of tumors that spontaneously develop in RB plus/minus mice by expressing an RB transgene under the control of the proopiomelanocortin and/or calcitonin promoters that are specific, respectively, for the pituitary melanotroph and thyroid C-cell lineages. Prevention of the rapidly progressing melanotroph or C-cell neoplasia will extend the life span of the mice, thus permitting a complete development of other tumors, including hematogenous metastasis to lungs and liver, which are more relevant to human diseases. Aim 2 is to test the hypothesis that targeted homozygous deletion of RB in a temporal, spatial, and cell-type specific manner will result in non-neuroendocrine neoplasia that mimic human cancers. Toward this end, novel models will be generated using Cre-loxP regulatory systems. Aim 3 is to test the hypothesis that the expression of RB abrogates progression and metastasis of RB-deficient tumors. Clinically feasible approaches for RB gene therapy will be applied to the animal models developed in Aims 1 and 2. We expect that the proposed studies will further substantiate the general theory of tumor suppression in immunocompetent animals. Thus, firm grounds for clinical trials on a significant subset of advanced human cancers will be established.